1. Field of the Invention
The present invention relates to lead-calcium-aluminum alloys used mainly in the manufacture of lead-acid battery grids of which so-called "maintenance-free" start-up batteries are composed.
2. Background Information
For some twenty years, the substitution of antimony with calcium and with tin in the lead making up these grids has led to a growing number of storage batteries of longer life and having a negligible consumption of electrolyte being put on the automobile market.
This is because calcium gives lead very useful mechanical properties and tin, while also hardening the lead, favors better energy transfer during the repeated charging/discharging cycles that the battery undergoes throughout its life.
Lead-calcium (Pb--Ca) and lead-calcium-tin (Pb--Ca--Sn) alloy manufacturers also add a small amount of aluminum intended to protect the baths for producing the alloy from extensive oxidation, which consumes mostly calcium at the expense of lead.
Lead-calcium-aluminum (Pb--Ca--Al) alloys, with or without tin, are used for manufacturing negative grids for batteries, while lead-calcium-aluminum-tin (Pb--Ca--Al--Sn) alloys are used for manufacturing positive grids.
The manufacture of a storage battery is a succession of a large number of operations, some of which are carried out between 60 and 80.degree. C. over periods ranging from twenty-four to forty-eight hours. These operations allow the alloy and the active substance to be given all the properties which they will subsequently have to have in order for the battery to operate correctly.
The development of new continuous processes for manufacturing battery grids has, moreover, given rise to a recent requirement for new materials meeting the specifications of new casting and expanding machines and allowing the production of positive or negative grids of high quality.
In the case of negative grids, the tendency is to develop lead-calcium-aluminum alloys having low contents of tin which rapidly hardens over time, thereby making it possible to increase the productivity of casting machines. They must furthermore have mechanical properties superior to those of the previous generation, so as to allow a reduction in the thickness of the grid and therefore a reduction in the weight of the battery without concomitantly impairing the mechanical integrity of the negative plate.
In the case of positive grids, the tendency is to develop lead-calcium-aluminum-tin alloys resistant to the corrosion and passivation phenomena which occur in the positive plate. They must also have a sufficiently high hardness or tensile strength so as to withstand, throughout the lifetime of the battery, the mechanical stresses which are exerted in the positive plate.
In point of fact, the manufacture of positive and negative battery grids requires special attention during the first step of pasting. During this operation, the freshly cast grid must in fact be sufficiently rigid not to deform under the pressure of applying the lead oxide paste which will fill the grid. Since the family of lead-calcium-aluminum-tin alloys is known for its room-temperature hardening properties, the rapidity of the kinetics of which process may vary, the battery manufacturer's know-how and that of his supplier are constantly being applied in order to optimize the efficiency and the quality of the manufacture. One method consists in providing temporary storage of the grids (individual grids or a set of rolls) so as to allow the hardening phenomenon time to develop. The use of a lead alloy having rapid hardening kinetics should allow the time for storing the grids to be reduced.
However, deformation, or even final fracture, of grids during their use in a battery remains one of the main quality problems with which battery manufacturers are confronted. This problem, which is particularly sensitive in the case of positive grids subjected to high mechanical and chemical stresses (corrosion in a sulfuric medium) requires the development of alloys which exhibit good corrosion resistance in a sulfuric medium and have mechanical properties which are high and remain constant over time.
However, it is recognized that one of the simplest means of reducing the sensitivity of thin battery grids to grain-boundary corrosion is to use, in their manufacture, alloys which solidify in a crystallographic structure containing small grains, since this type of structure is reputed to be less sensitive to grain-boundary corrosion.
In order to solve these problems, the addition of barium in lead-calcium-tin alloys has already formed the subject of prior studies mentioned, for example, in Patents FR-A-851,686, DE-2,611,575, DE-2,619,113, EP-A-040,951, DE-2,921,290, GB-1,597,270 and GB-1,304,095. However, although it is actually mentioned that the presence of barium substantially improves the mechanical integrity of cast grids (mechanical strength and creep strength) while not degrading their corrosion behavior, the relative barium contents by weight proposed are always greater than 0.025%. For example, from 0.05 to 0.5% barium is found in the case of DE-2,619,113 and DE-2,611,575; from 0.026 to 0.044% as a complete calcium substitute is found in the case of GB-1,597,270 and DE-2,921,290; and from 0.025 to 0.1% barium with, systematically, strontium between 0.15 and 0.4% and a calcium content of from 0.03 to 0.04% are found in the case of EP-A-040,951 and/or are combined with other additions (magnesium and lithium).
Incidentally, it may be pointed out that Patent FR-A-851,686 recommends an alloy having very high barium contents for producing railroad bearings. For example, it refers to a lead-calcium-barium alloy containing from 0.1% to 2% calcium, from 0.5% to 10% tin and from 0.02% to 0.1% barium. This patent mentions that this alloy has very good corrosion resistance properties in organic medium (oils).
The only reference to alloys having low barium contents is found in GB-1,304,095 which mentions, generally, the beneficial effects of adding from 0.001 to 1% barium, but in an alloy not containing calcium.
An exhaustive study of the prior documents shows that most of the studies carried out in the past related to alloys with or without calcium and having high barium contents (greater than 0.02%).